Eighty percent of the world?s population lives in low and middle income countries (LMIC) with few mental health resources, resulting in a substantial mental health treatment gap. Growing evidence indicates that evidence- based mental health treatments can be delivered in LMIC using a task-sharing approach, in which non- professionals deliver care under supervision. Very limited research, however, focuses on how to embed, support, and effectively deliver these treatments within existing, government-supported systems in which they could be scaled up to population-level. With LMIC governments typically spending <2% of their national budget on mental health, innovative and low-cost options are needed for intervention delivery and for implementation support. Building and Sustaining Interventions for Children (BASIC): Task-sharing mental health care in low-resource settings builds on our 15-year history of collaborations with research partners in Kenya, prior NIH-funded work that identified mental health needs of orphaned children in LMIC, and iterative and collaborative intervention adaptation and testing using a task-sharing approach, to address these needs. In BASIC, we test the implementation of Trauma-focused Cognitive Behavioral Therapy (TF-CBT), delivered via task-sharing, in two governmental sectors prioritized by our Kenyan partners as potential options for scale up? Education and Health Extension. The recent devolvement of the Kenyan government (leading to more local decision-making), the launch of a National Mental Health Policy, and our Kenyan partners? empowerment work building enthusiasm for TF-CBT are converging to create a local climate in which BASIC could become part of the county plan, if evidence-based guidance for implementation, using mostly existing resources, existed. We test mental health treatment delivery in Education (via teacher delivery) and Health Extension (via community health volunteers) with the goal of identifying implementation practices and policies (IPPs) that explain implementation outcomes. This stepped wedge cluster randomized trial includes 40 schools and the 40 surrounding villages (120 lay counselors in each) who provide TF-CBT to 1,280 youth. We use a novel method, qualitative comparative analyses (QCA), that holds potential for substantially advancing the field of implementation science. QCA leverages the rigor of quantitative approaches and the detail of qualitative approaches, and allows for complex causality and equifinality (i.e., an outcome can be reached by multiple means). Study aims are: 1) Identify actionable IPPs that predict adoption (delivery) and fidelity (high- quality delivery) after 10 sites in sector implement TF-CBT. Use identified IPPs to (Aim 1a) guide implementation planning support for subsequent sites and to (Aim 1b) generate testable hypotheses about IPPs as causal mechanisms; 2) Test mechanisms of implementation success in both sectors; and 3) Test TF-CBT effectiveness (i.e., mental health outcomes; functioning) and cost in both sectors. This research has important implications for implementing an EBT in low-resource settings, including the US.